Styrene synthesis over iron oxide catalysts: from single crystal model system to real catalysts

Surface science methods originating from analysis of noble metal catalysts are increasingly applied to metal oxides. These methods provide direct access to fundamental structural properties and phase equilibria governing the catalytic properties of metal oxide surfaces. However, no systematic way existed so far for transferring this knowledge to technical catalysts. The aim of this paper is to combine surface science with chemical engineering methods to bridge this gap. Styrene synthesis over pure and K-doped iron oxides is used as an example to develop and to explain the methodology. Single crystal films (SCF), grown epitaxially on a Pt-carrier are considered as ideal model surfaces. Comprehensive UHV analyses yield the structural properties of SCF as well as their interaction with relevant components of the reaction mixture. Their results are combined with conversion experiments to derive a mechanistic catalyst model along with quantitative information on the reaction rates. The activity of SCF as well as their phase transitions under reactive conditions can be described with a continuum model depending on the macroscopic properties of the system. This model forms the crucial link towards technical catalysts. It is shown that the behaviour of a powder catalyst can be described as a superposition of the above kinetic model and an appropriate porous model. In this paper we review the developed methodology and conclude with the evaluation of the concept.     

Adsorption of Gases on Complex Solid Surfaces

During the last thirty years the research field of surface science with its various disciplines has progressively played a more and more important role in the field of catalysis. The main focus of attention for a long time was research on metal surfaces, on which, in time, the whole spectrum of developed surface analytical methods was applied. This led to a better understanding of the mechanisms of catalytic reaction, such as the synthesis of ammonia and the oxidation of CO, especially through the work of Gerhard Ertl.^{[1, 2]} In contrast to clean metal surfaces, surfaces of real catalysts are complex entities, the structures of which can have a strong influence on the processes occurring on the surface. Thus, it seems logical to employ the typical structural characteristics and the morphology of the catalytic surface as guidelines in the investigation of complex model systems. In this review the preparation, and structural and electronic characterization of such model systems will be dicussed. Clean surfaces of catalytically active oxides, as well as model systems for dispered transition metal/support catalysts will be characterized in terms of their morphology and electronic structure as well as their adsorption and reaction capabilities.     

Heterogeneous catalysis: enigmas, illusions, challenges, realities, and emergent strategies of design

Predominantly this article deals with the question of how to design new solid catalysts for a variety of industrial and laboratory-orientated purposes. A generally applicable strategy, illustrated by numerous examples, is made possible based on the use of nanoporous materials on to the (high-area) inner surfaces of which well-defined (experimentally and computationally) active centers are placed in a spatially separated fashion. Such single-site catalysts, which have much in common with metal-centered homogenous catalysts and enzymes, enable a wide range of new catalysts to be designed for a variety of selective oxidations, hydrogenations, hydrations and hydrodewaxing, and other reactions that the "greening" of industrial processes demand. Examples are given of new shape-selective, regio-selective, and enantioselective catalysts, many of which operate under mild, environmentally benign conditions. Also considered are some of the reasons why detailed studies of adsorption and stoichiometric reactions at single-crystal surfaces have, disappointingly, not hitherto paved the way to the design and production of many new heterogeneous catalysts. Recent work of a theoretical and high-throughout nature, allied to some experimental studies of well-chosen model systems, holds promise for the identification of new catalysts for simple, but industrially important reactions.     

Single carbon dioxide molecules on surfaces studied by low-temperature scanning tunneling microscopy

The chemical conversion of carbon dioxide (\(\hbox {CO}_2\)) has been intensively studied because the molecule is responsible for global warming. Rational design of catalysts plays an important role in converting \(\hbox {CO}_2\) into value-added compounds. Understanding the interaction between \(\hbox {CO}_2\) and surfaces of catalysts is a prerequisite to preparing high-performance catalysts. This review focuses on the investigations of \(\hbox {CO}_2\) molecules on single crystalline surfaces studied by low-temperature scanning tunneling microscopy. Molecular adsorption, diffusion, and conversion on metal surfaces, metal oxide surfaces, and surfaces decorated by metal-organic frameworks are summarized.     

Cu-ZnO催化剂上水活化的第一性原理研究

采用密度泛函理论计算研究了水在Cu-ZnO催化剂表面上不同位点的解离过程. 结果发现, 水在纯Cu密堆积面和台阶面解离能垒都较高; 而在负载的ZnO薄膜上, 由于水解离过程能垒较低并且反应约为热中性, 水将会在表面上部分解离并达到动力学平衡. Cu-ZnO界面被确定为水解离的活性中心. 水解离后产生的H原子和羟基均可以较大吸附能吸附在界面处, 并且界面处的类似台阶结构大大降低了解离能垒, 从而使得水的解离可自发进行. 另外, H原子和羟基在ZnO薄膜表面可以较低的能垒扩散, 因此水解离活性位点可以持续催化后续解离过程. 该结果深化了对水在Cu-ZnO催化剂表面活化过程的认识.     

乙烯基乙酸酯合成钯-金催化剂中金的助催化作用

硅胶负载的钯-金双金属催化剂是乙烯乙酰氧基化制乙烯基乙酸酯(VA)的高选择性催化剂,本文应用平面和负载纳米颗粒模型催化剂体系研究金的助催化作用,应用低能离子散射谱、低能电子衍射、X射线光电子能谱、反射红外吸收光谱及程序升温脱附等技术表征这些模型催化剂.结果表明,金的主要助催化作用是隔离催化剂表面的催化活性钯原子,形成孤立的钯活性中心,从而大大抑制或消除反应物和/或产物在毗邻多原子钯中心上的深度分解,提高VA合成的选择性及活性.同时由于形成了孤立的钯原子活性中心,反应副产物或中间物之一的一氧化碳吸附较弱,避免了催化剂表面的一氧化碳中毒,进而提高催化活性.     

Effective way of modeling chemical catalysis: Empirical valence bond picture of role of solvent and catalyst in alkylation reactions

A general methodology for the study of chemical catalysis is presented and demonstrated in a study of Friedel–Crafts‐type alkylation reactions that are constrained to collinear configurations. Ab initio potential energy surfaces in solution and relevant experimental results are used to calibrate general empirical valence bond (EVB) potential surfaces for studies of such reactions. The EVB surfaces allow one to interpolate the ab initio results to studies of the effect of different solvents, substituents, and catalysts on the alkylation reactions. This implicit approach introduces such effects by shifting the diagonal energies of the corresponding resonance structures. Such an EVB/shift approach appears valuable for assessing the effects of different catalysts and solvents on complex chemical reactions. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 607–625, 2000     

XPS characterization of the reduction and synthesis behaviour of Co/Mn oxide catalysts for Fischer-Tropsch synthesis

Summary The surface composition of Co/Mn oxide catalysts after calcination, reduction and during CO hydrogenation experiments were investigated by XPS. The bulk changes during reduction were studied by Temperature Programmed Reduction (TPR). The calcined catalysts showed Co/Mn oxides of different compositions in their surfaces. The Co surface concentration, of the catalysts with high Co content, decreased after calcination compared to the bulk composition, but after reduction the bulk concentration was almost reached again. The catalysts with low Co content showed no decrease in the surface concentrations after calcination. Significant differences in surface concentrations for the catalysts Co20 and Co5 were observed by analysing the Co 2p and 3p levels, respectively; these can be explained by an internal reduction model. After reduction the sample Co100 was completely reduced to metallic cobalt. In the manganese-containing catalysts, after in-situ reduction, Co³⁺ and Co²⁺ were found; all manganese was reduced to Mn²⁺. A comparison of the results of the in-situ reduction and the TPR profiles led to the development of a so called internal reduction model. This model assumes migration of the Co²⁺ ions to the reduction front in inner layers of the catalysts, where they will be reduced to metallic cobalt; this is enriched in the bulk and cannot be found at the surface. The manganese matrix stabilizes the surface oxide layer so that all catalysts exhibited Co/Mn spinels in the surface. Synthesis experiments in the reaction chamber of the XPS apparatus did not lead to changes of the catalyst surfaces as a function of the reaction pressure, synthesis time or synthesis gas composition. The differences in the synthesis behaviour observed for the catalysts must be due to other effects, (i.e. a change in adsorption of hydrogen connected with a change in hydrogenation activity or the different cobalt concentrations).     

低温加氢催化剂的设计:理论与实践(英文)

简要总结了我们在C=C及C=O双键低温加氢双金属催化剂方面的最新研究成果.首先,我们以环己烯加氢为探针反应,证明了平行使用多种研究手段的重要性,包括单晶表面的基础研究与DFT计算,多晶表面的合成与表征,负载型催化剂的制备与性能测试等.其次,总结了双金属催化剂在其他加氢反应,如丙烯醛C=O双键的选择性加氢,苯的低温加氢,以及乙炔的选择性加氢等反应中的应用.最后,讨论了利用金属碳化物代替贵金属Pt以减少双金属催化剂中Pt用量的可能性.     

催化剂活性位本质和构效关系的模型催化研究

明确催化剂的活性位本质和构建多相催化的结构和反应性能之间的准确关系是催化基础研究的重点,表面科学研究基于丰富的表征测试手段能够较好地在分子原子水平测定表面结构以明确催化剂活性位本质,并通过高压原位反应池测定相关催化反应性能,获得较可靠的催化剂构效关系。本文简要总结了近年来本人参与的几个模型催化研究例子,包括贵金属表面上CO和烷烃催化氧化的活性表面、纳米Au膜的制备和CO氧化的催化活性位、VO_x/Pt(111)上丙烷氧化的协同作用、Au Pd合金上醋酸乙烯酯合成Au的助催化作用、模型氧化物上纳米Pt的庚烷脱氢环化制甲苯的粒径关系等,以及相关模型催化研究技术的进展。     

Structure and reactivity of Ru nanoparticles supported on modified graphite surfaces: a study of the model catalysts for ammonia synthesis

Supported ruthenium metal catalysts have higher activity than traditional iron-based catalysts used industrially for ammonia synthesis. A study of a model Ru/C catalyst was carried out to advance the understanding of structure and reactivity correlations in this structure-sensitive reaction where dinitrogen dissociation is the rate-limiting step. Ru particles were grown by chemical vapor deposition (CVD) via a Ru(3)(CO)(12) precursor on a highly oriented pyrolytic graphite (HOPG) surface modified with one-atomic-layer-deep holes mimicking activated carbon support. Scanning tunneling microscopy (STM) has been used to investigate the growth, structure, and morphology of the Ru particles. Thermal desorption of dissociatively adsorbed nitrogen has been used to evaluate the reactivity of the Ru/HOPG model catalysts. Two different Ru particle structures with different reactivities to N(2) dissociation have been identified. The initial sticking coefficient for N(2) dissociative adsorption on the Ru/HOPG model catalysts is approximately 10(-6), 4 orders larger compared to that of Ru single-crystal surfaces.     

DFT Studies of Palladium Model Catalysts: Structure and Size Effects

An important task for theory is the multi-scale modeling of catalytic properties of nanocrystallites with sizes ranging from clusters of few metal atoms to particles consisting of 10³–10⁴ atoms. To explore catalytic properties of nanosized metal catalysts, we developed an approach based on three-dimensional symmetric model clusters of 1–2 nm (~100 metal atoms) with fcc structure, terminated by low-index surfaces. With this modeling technique one is able to describe at an accurate DFT level various catalytic and adsorption properties of metal nanoparticles in quantitative agreement with experimental studies of model catalysts deposited on thin oxide films. Metal nanocrystallites exhibit properties that can significantly vary with their size and shape.     

纳米Pd和Au催化CO2还原粒径效应的密度泛函理论计算研究

以可再生能源为能量来源,在水溶液中进行的光(电)催化CO₂还原生成高附加值化学品和燃料是解决能源危机与环境污染的有效途径之一.CO是一种简单却很重要的CO₂还原产物,它可以作为水煤气变换反应与费托合成的重要原料.具有较高CO选择性的贵金属纳米颗粒催化剂(如Au和Pd)一直受到研究者的广泛关注.一般来说,金属颗粒催化剂的催化性能与粒径大小密切相关,即所谓的粒径效应.然而在实际的理论计算研究中,由于受到计算能力的限制,催化剂模型都仅局限于简单的周期性模型或小的金属团簇模型,无法准确描述真实颗粒上复杂的反应位点的性质,导致了对催化行为的误解.因此,建立更加真实的颗粒模型对探究纳米颗粒催化剂上活性位点的性质,解释其粒径效应至关重要.本文旨在阐述Au与Pd纳米颗粒催化剂不同活性位点上CO₂还原反应与产H2副反应的竞争机制,并解释Au与Pd纳米颗粒催化剂在CO₂电还原中表现出不同粒径效应的原因.本文基于密度泛函理论,采用VASP软件,BEEF-vdW泛函进行计算.分别建立了原子数为55,147,309和561的颗粒模型和高CO^*覆盖度模型,避免了传统周期性模型的局限性,探究了金属颗粒催化剂不同反应位点上的CO选择性.结果表明,对于颗粒模型来说,(100)位点对CO的选择性优于边缘位点;但对于周期性模型来说,Au(211)对CO的选择性则优于Au(100).产生这种反差的主要原因在于Au颗粒的边缘位点对H*的吸附过强.通过对比,我们直观地展现了颗粒模型上平面位点和Edge位点与相对应的周期性模型上CO选择性的区别,突出了模型选择对揭示活性位点性质的重要性.在此基础上,通过计算理论CO法拉第效率,发现Au颗粒随着粒径的减小,CO选择性降低,与实验的趋势一致.对于Pd催化剂来说,低覆盖度模型无法正确预测活性位点的性质;而高CO覆盖度的情况下,Pd颗粒的边缘位点对COOH^*吸附能更强,这是导致边缘位点上CO选择性更高的主要原因.同样通过计算理论CO法拉第效率,发现随着粒径的减小,Pd颗粒上CO选择性升高.本文不仅成功揭示了Au与Pd颗粒催化剂上活性位点的性质,对粒径效应做出了合理解释,也强调了合理的计算模型是理论研究的基础.     

A versatile electroless approach to controlled modification of Sb on Pt surfaces towards efficient electrocatalysis of formic acid

A new facile approach towards developing superior Pt-based catalysts for HCOOH electrooxidation has been proposed, which is exemplified with a mimetic underpotential deposition (MUPD) of Sb on Pt surfaces to attain a favorable coverage. Suitable Sb modification was achieved simply through immersing a bulk Pt electrode or dispersing Pt/C powders in a Sb(III) solution mixed with ascorbic acid (AA). AA serves as the mild reducing agent to ensure freshly reduced Pt surfaces for Sb modification, as demonstrated by the negatively shifted open circuit potential. The catalytic activity towards HCOOH electrooxidation on the above Sb-modified Pt/C catalyst far exceeds that on commercial Pt–Ru/C or Sb-modified Pt/C through traditional irreversible adsorption. This electroless approach is generally applicable to all types of Pt surfaces, in particular suited for upgrading Pt/C for practical anode catalysts of direct formic acid fuel cells.     

铂基氧还原催化剂在活性和稳定性方面的挑战

在质子交换膜燃料电池（PEMFC）中,由于阴极氧还原反应（ORR）速率缓慢,因此开发高效的ORR催化剂是实现燃料电池商业化的关键. 世界各地的研究人员在提高催化剂活性和耐久性方面做出了不懈的努力. 目前,铂基催化剂仍然是商业应用上的首选,为开发实用的低铂氧还原催化剂,研究人员开展了大量的研究. 本文说明了ORR反应遇到的挑战,并介绍了近年来铂基氧还原催化剂的研究进展,具体包括ORR机理、铂核壳结构、一维纳米Pt催化剂和其他的代表性工作.     

Mechanistic comparison of ruthenium olefin metathesis catalysts: DFT insight into relative reactivity and decomposition behavior

The reaction mechanism and substrate-induced decomposition behavior of three ruthenium olefin metathesis catalysts, viz. first- and second-generation catalysts and the recently developed Phoban catalyst (“Phobcat”) are compared by constructing ΔG surfaces at 298.15 K and 1 atm for the complete ligand systems. From these calculations fundamental insight is gained into the reactivity and stability observed experimentally for the three catalysts. In particular, the higher conversions obtained for the first-generation derived Phobcat catalyst, compared to conventional first-generation catalysts, is attributed to its similarity to the second-generation catalysts instead of first-generation catalyst. Important differences between the calculated ΔG surfaces and previously reported total electronic energy (ΔE) surfaces for the metathesis mechanism with complete ligand complexes are discussed.     

Metallic nature and surface diffusion of CO adsorbed on Ru nanoparticles in aqueous media: A 13C NMR study

We report the first observation of the 13C nuclear magnetic resonance spectroscopy (NMR) of 13CO, adsorbed from 13CO saturated 0.5 M sulfuric acid solutions, onto the surfaces of commercial Ru-black nanoparticles. The 13C NMR spectra consist of a symmetrically broadened peak having a large isotropic shift as compared to CO adsorbed onto supported Ru catalysts. The variation of the spin-lattice relaxation rate follows Korringa behavior, indicating the metallic nature of adsorbed CO, in addition to varying across the spectrum in a Korringa-like manner. Motional narrowing of the NMR spectrum at higher temperatures, together with an additional contribution to the spin-lattice relaxation rate, indicate that adsorbed CO undergoes rapid diffusion on the particle surfaces. A two-band model analysis of the NMR results indicates that the CO adsorption bond is weaker on Ru as compared to either Pt or Pd. This is also supported by a reduction in the activation energy for CO diffusion on Ru vs either Pt or Pd nanoparticles.     

Synergetic effect of surface and subsurface Ni species at Pt-Ni bimetallic catalysts for CO oxidation

Various well-defined Ni-Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO(1-x)/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt-Ni nanoparticles via activation in H(2) at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt-Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O(2), producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.     

Preparation of surface organometallic catalysts by gas-phase ligand stripping and reactive landing of mass-selected ions

Organometallic complexes immobilized on surfaces combine the high selectivity of homogeneous catalysts with the ease of separation of catalyst from products attainable with heterogeneous catalysts. Here we report a novel approach for the highly controlled preparation of surface organometallic catalysts by gas-phase ligand stripping combined with reactive landing of mass-selected ions onto self-assembled monolayer surfaces. Collision-induced dissociation is used to generate highly reactive undercoordinated metal complexes in the gas-phase for subsequent surface immobilization. Complexes with an open coordination shell around the metal center are demonstrated to show enhanced activity towards reactive landing in comparison to fully ligated species. In situ TOF-SIMS analysis indicates that the immobilized complexes exhibit behavior consistent with catalytic activity when exposed to gaseous reagents.     

Surface Defects Activating New Reaction Paths: Formation of Formate during Methanol Oxidation on Ru(0001)

Methanol was co‐adsorbed with oxygen on Ru(0001) under conditions approaching those of real catalysts: at room temperature and at relatively high pressures and exposures, together with a comparative analysis of flat and defective surfaces. To clarify reaction routes, parallel exposures to formaldehyde and oxygen have also been analyzed. It is found that for both mixtures of gases, a new reaction path is activated on defective surfaces, in which methanol is oxidized to formate. Furthermore, at variance with pure methanol adsorption, apart from CO, various intermediates are observed in both flat and defective surfaces. On flat surfaces, formaldehyde and formyl are recognized whereas on defective ones methoxy and formate are detected. A model involving steering effects is presented, which accounts for the activity of surface defects towards the synthesis of formate.